The present invention pertains to cooling systems and, more particularly, to a fan assembly incorporating blades which may be adjusted to vary the pitch thereof in order to alter the air flow characteristics of the fan assembly. The invention is also directed to an improved blade unit for use in such a fan assembly, as well as a method of constructing the fan assembly.
Providing a fan assembly including a plurality of circumferentially spaced blades for developing a flow of air for cooling purposes is well known. Such fan assemblies are widely used in numerous fields, such as for cooling heat generating devices. For example, in the automotive art, fan assemblies are commonly used for engine cooling purposes. More specifically, a fan assembly is generally attached to a block of the internal combustion engine and is driven by the engine through a sheave and belt drive arrangement. The fan assembly mainly delivers a flow of air across a radiator and is incorporated as part of an overall, thermostatically controlled engine cooling system.
Since the fan assembly is driven by the engine, the rotating speed of the fan blades tracks the rpm's of the engine. However, the drive to the fan assembly typically incorporates a clutching mechanism such that the fan assembly either assumes an off condition, wherein no air flow is generated by the fan assembly, or an on condition, wherein the fan assembly is driven at a maximum rate established by the prevailing engine speed. With such an arrangement, a considerable initial load is placed on the drive system, particularly the belts, when the clutching mechanism is activated while the engine is running at a high rate of speed. Another problem associated with such typical engine cooling arrangements is that there is no control over the amount of power the fan assembly will use. Instead, the horsepower draw from the engine is always locked into a predetermined power versus fan speed relationship, i.e., power draw is cubic in relation to the rotating speed of the fan, while accounting for air density and temperature factors. This is particularly disadvantageous when cooling needs are low, but the fan assembly is still activated at a high speed. Furthermore, engaging the fan assembly can be a major source of noise, especially at low engine rpm's. For instance, when the engine is idling, noise generated by the engagement of the fan can be quite disturbing, with the majority of the noise being produced by the engagement of the frictional elements of the clutching mechanism.
Mainly due to the problems outlined above, variable speed fan assemblies, such as those incorporating viscous and eddy current-type fan clutches, and variable pitch fan assemblies have been developed. In general, variable speed fan assemblies are advantageous as the operating speed of the fan blades can be better correlated to the degree of cooling required. Of course, even variable speed fan assemblies still only provide a set air flow rate at any given fan operating speed. In addition, viscous drives cannot generally provide a fully "off" condition or a "maximum" air flow condition since they are continuously slipping. Here, variable pitch fan assemblies can be advantageously used since the pitch of the blades can be adjusted according to prevailing cooling requirements such that a reduced power draw from the engine can be achieved. Furthermore, variable pitch fan assemblies can be initially engaged in a smooth and quiet manner, even at low engine speeds, and can readily assume both off and full on conditions.
Although variable pitch fan assemblies can be adjusted to establish further enhanced air flow characteristics, problems exist in current designs, particularly regarding the ease of maintenance. For example, due to the connections needed to alter the pitch of the blades during use, the blades cannot be readily detached from the remainder of the fan assembly. In addition, the blades of prior art variable pitch fan assemblies have stems rotatably supported in outer housing openings only, typically through bearings that are connected to the stems. This arrangement can result in considerable force being exerted on concentrated portions of the housing, particularly during times of high speed operation. Furthermore, given that considerable force can act on the fan blades which tend to draw the blades radially outward, loosening and even separation of the blade stem from the bearings and housing can occur.
Based on the above, there exists a need for a fan assembly which is designed to establish optimal air flow rates, exhibits superior maintenance characteristics and provides an enhanced fan blade construction and mounting arrangement.